In modern communication devices such as smartphones and tablet computers an increasing number of radio interfaces operate in parallel. Some radio interfaces like those defined by the family of 3rd Generation Partnership Project (3GPP) specifications are enabled to co-operate in such a way that there is no need for a transmission on one radio interface while another radio interface is receiving. In this way, in-device co-existence issues of the radio interfaces (e.g., in terms of mutual interference) can be avoided.
On the other hand there also exist radio interfaces which per se are not co-operative. For example, Bluetooth, Wideband Local Area Network (WLAN) and Global Navigation Satellite System (GNSS) radio interfaces will on a physical layer level typically not co-operate in any way with the radio interfaces defined in the 3GPP specifications.
Many use cases such as WLAN tethering or a Voice over Internet Protocol (VoIP) call with a Bluetooth headset may require a simultaneous operation of the WLAN or Bluetooth radio interface and of a cellular radio interface. Both WLAN and Bluetooth presently operate in the so-called ISM band (2400 MHz-2483 MHz), and beyond. The Frequency Division Duplex (FDD) mode of the 3GPP Long Term Evolution (LTE) radio interface may be operated in band 7 (2500 MHz-2570 MHz). The Time Division Duplex (TDD) mode of the LTE radio interface may be operated in band 40 (2300 MHz-2400 MHz) or band 41 (2496 MHz-2690 MHz). Due to the close proximity of LTE bands 7, 40 and 41 on the one hand and the ISM band on the other hand, measures have to be taken to limit the interference between the radio interfaces.
Similar interference problems occur between GNSS bands L1 and E1 (1575.42 MHz) and second order harmonics of an LTE FDD uplink signal in band 13 (777 MHz-787 MHz) or band 14 (788 MHz-798 MHz). GNSS applications typically operate at an extremely low signalling level that requires long correlation and averaging times to capture the satellite signals. As will be appreciated, an intermittent LTE transmission may even further extend the GNSS correlation and averaging times.
To combat such interference problems, filtering has been suggested for radio link protection in communication devices that integrate multiple radio interfaces. It has, however, been found that pure filtering is inadequate in case of only narrow gaps between the involved bands and because of the entailed extreme filter requirements. Another solution would be frequency evasion for certain use cases such as an LTE voice call with a Bluetooth headset. In such a situation, however, a significant portion of a network operator's frequency band would not be available for many use cases. Furthermore, additional signalling with the network side would be required. A still further approach for radio link protection could be a non-overlapping scheduling of the transmission and reception activities of different radio interfaces. Such a solution would on the other hand significantly restrict the scheduling possibilities and would hardly be appropriate for FDD solutions that do not have a natural uplink/downlink separation in the time domain.